1. Technical Field of the Invention
The present invention relates generally to the field of electronics. More specifically, the present invention provides radio frequency (RF) connectors and electronics housings or packages employing one or more inventive RF connector(s). RF connector(s) disclosed herein utilize a ground spring to achieve improved conductivity of the ground signal by making a plurality of contacts with a ferrule member of the RF connector's hermetic feedthru and a plurality of contacts with the electronics housing or package at points adjacent to an air dielectric. Ground springs used in connection with the RF connectors of the present invention maintain predetermined spring properties under compression and/or extreme environmental conditions, including thermal fluctuations, and therefore may be suitably employed in aircraft and spacecraft.
2. Description of the Related Art
Electronic components are used in countless applications in a wide variety of environments. Such components are subject to faulty operation, degradation, and corrosion resulting from contact with dust, water vapor, gases, and the like, as well as from high temperature and/or pressure conditions. In order to protect electronic components from such harsh conditions of the operating environment, they are generally, although not exclusively, hermetically sealed within an electronics housing or package that is desirably constructed from materials that meet application specific requirements for density, thermal expansion, thermal conductivity, mechanical strength, and the like. For example, electronics packages used in aircraft and spacecraft applications must be lightweight and are therefore constructed from low density materials such as aluminum or titanium alloys.
Commonly, electronic components on the inside of an electronics housing or package are in electrical contact with components on the exterior of the package by way of an electrical connector, such as an RF connector, that incorporates a hermetic feedthru to maintain the integrity of the electronics housing or package interior. The basic elements of representative prior art “spark plug” and “field replaceable” RF connectors are presented in FIGS. 1A and 1B, respectively.
FIG. 1A depicts a typical “spark plug” type RF connector 10 with a hollow, exteriorly threaded stainless steel shell 12 having a KOVAR™ glass-to-metal feedthru 14 affixed thereto by brazing at elevated temperature. Shell 12 also houses a teflon (or other insulating material) insert 16 having a pin socket 18 disposed therein at each longitudinal end. A connector pin 20, generally formed of an iron-based metal, inserts into pin socket 18. A teflon member 22 surrounds connector pin 20 in longitudinal juxtaposition to shell 12, and a double knife edge seal ring 24 is disposed in circumferential juxtaposition to shell 12. Ring 24 is formed of an iron-based metal, such as KOVAR™ or stainless steel, and is optionally coated with silver.
To affix RF connector 10 to an interiorly threaded electronics housing or package 26, torque (approximately 25 in-lbs) is applied to RF connector 10. This force causes seal ring 24 to slightly cut into both RF connector 10 and electronics housing or package 26, thereby creating a seal. To insure that RF connector 10 does not back out of electronics housing or package 26 during transport or use, an edge 28 of an RF connector 10-electronics housing or package 26 assembly is soldered about the circumference of RF connector 10. For this purpose, gold plating is optionally used to improve the wetting properties of the solder.
Because of the differing thermal expansion properties of the electronics housing or package and prior art “spark plug” type RF connector, i.e. the externally threaded iron-based metal and the internally threaded aluminum metal, the seal between these components does not reliably maintain its hermeticity. The two dissimilar metals are in intimate contact at ambient temperature; however, since aluminum has a higher expansion rate than does either KOVAR™ or stainless steel, temperatures lower than ambient cause package 26 to squeeze RF connector 10, while temperatures higher than ambient produce a separation between those components. Such phenomena result in fatigue of the solder joint during thermal cycling and cause less than intimate contact between seal ring 24 and electronics housing or package 26 as well as between seal ring 24 and RF connector 10.
Furthermore, the external solder application at 28 prevents RF connector 10 backout by providing a mechanical lock between the components, but because of material fatigue this solder joint also does not form a reliable hermetic seal. And, this RF connector is not field replaceable because removal of the connector compromises the hermeticity of the package and breaks the rigid connection to the end of the pin located inside the package. That is, RF connector 10 cannot be replaced in the field without a high risk of compromising the integrity of electronics housing or package 26 circuitry.
Significant in regards to the presently disclosed invention, the electrical performance of RF connector 10 suffers as a result of temporal disparity owing to differences in lengths of the conductance path of the RF signal and the ground signal to electronics housing or package 26. While the RF signal follows an essentially straight line path through RF connector 10 into electronics housing or package 26 by way of the pin member 20, the ground path must run along the outer surface of teflon insert 16, the outer surface of the glass portion of feedthru 14, the outer surface of teflon member 22, through seal ring 24 into electronics housing or package 26 and about the periphery of the interior of package 26 to the ground location within the electronics housing or package. The resulting ground lag impacts signal gain and loss characteristics, thereby affecting the signal-to-noise ratio. This problem is exacerbated as higher frequency signals are employed.
FIG. 1B depicts a prior art “field replaceable” RF connector 30, which includes an exteriorly threaded, replaceable portion 32 formed of stainless steel. A KOVAR™ glass-to-metal feedthru 34 is soldered into a cavity 36 in an aluminum electronics housing or package 38 at one or more solder locations 40. Replaceable portion 32 is torqued into an interiorly threaded aluminum portion 42.
As described above with respect to the prior art “spark plug” type RF connector of FIG. 1A, seals using field replaceable connectors 30 are hermetic at ambient temperature, but because of the approximately 4:1 thermal expansion mismatch between KOVAR™ and aluminum, the hermeticity of the KOVAR™-aluminum solder seal fails due to metal fatigue with repeated temperature variations. Moreover, connector 30 does not meet military field replaceability standards because an iron-based metal part may be threaded into aluminum only once, because that operation impacts subsequent torque applications by displacing the aluminum in the threaded area.
In an attempt to overcome limitations in prior art RF connectors owing to metal mismatching and fatigue of solder connections, laser welding, rather than soldering, of RF connectors has been utilized to achieve reliable hermetic packaging. For example, RF connectors have been designed to be laser welded directly into an electronics housing or package thus eliminating hermetic failure due to solder joint fatigue. See, e.g., U.S. Pat. No. 5,298,683 to Taylor. Laser welding provides further advantages because the heating is localized at the weld, which permits the enclosure to be welded without damage to the delicate instruments and electronics installed inside. The localized heating also precludes weld induced thermal distortion of the enclosure and obviates the introduction of flux or other contaminants into the enclosure. And, the laser welding process lends itself well to automation for high production rates and low cost.
A limitation of RF connectors stemming from the use of laser welding that has not been adequately addressed in the art, however, is that laser welds, unlike solder joints, do not form a suitable ground path between an RF connector and the electronics housing or package to which it is welded. Thus, the ground lag seen in prior art RF connectors, as described above, that results from differences in signal and ground path lengths significantly compromises the RF connector's signal to noise ratio. What is needed in the art, therefore, are RF connectors having improved ground path conductivity properties.